'\" t
.TH "SD_JOURNAL_QUERY_UNIQUE" "3" "" "systemd 257" "sd_journal_query_unique"
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.SH "NAME"
sd_journal_query_unique, sd_journal_enumerate_unique, sd_journal_enumerate_available_unique, sd_journal_restart_unique, SD_JOURNAL_FOREACH_UNIQUE \- Read unique data fields from the journal
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-journal\&.h>
.fi
.ft
.HP \w'int\ sd_journal_query_unique('u
.BI "int sd_journal_query_unique(sd_journal\ *" "j" ", const\ char\ *" "field" ");"
.HP \w'int\ sd_journal_enumerate_available_unique('u
.BI "int sd_journal_enumerate_available_unique(sd_journal\ *" "j" ", const\ void\ **" "data" ", size_t\ *" "length" ");"
.HP \w'int\ sd_journal_enumerate_unique('u
.BI "int sd_journal_enumerate_unique(sd_journal\ *" "j" ", const\ void\ **" "data" ", size_t\ *" "length" ");"
.HP \w'void\ sd_journal_restart_unique('u
.BI "void sd_journal_restart_unique(sd_journal\ *" "j" ");"
.HP \w'SD_JOURNAL_FOREACH_UNIQUE('u
.BI "SD_JOURNAL_FOREACH_UNIQUE(sd_journal\ *" "j" ", const\ void\ *" "data" ", size_t\ " "length" ");"
.SH "DESCRIPTION"
.PP
\fBsd_journal_query_unique()\fR
queries the journal for all unique values the specified field can take\&. It takes two arguments: the journal to query and the field name to look for\&. Well\-known field names are listed on
\fBsystemd.journal-fields\fR(7), but any field can be specified\&. Field names must be specified without a trailing
"="\&. After this function has been executed successfully the field values may be queried using
\fBsd_journal_enumerate_unique()\fR
and
\fBsd_journal_enumerate_available_unique()\fR\&. Invoking one of those calls will change the field name being queried and reset the enumeration index to the first field value that matches\&.
.PP
\fBsd_journal_enumerate_unique()\fR
may be used to iterate through all data fields which match the previously selected field name as set with
\fBsd_journal_query_unique()\fR\&. On each invocation the next field data matching the field name is returned\&. The order of the returned data fields is not defined\&. It takes three arguments: the journal object, plus a pair of pointers to pointer/size variables where the data object and its size shall be stored\&. The returned data is in a read\-only memory map and is only valid until the next invocation of
\fBsd_journal_enumerate_unique()\fR\&. Note that the data returned will be prefixed with the field name and
"="\&. Note that this call is subject to the data field size threshold as controlled by
\fBsd_journal_set_data_threshold()\fR
and only the initial part of the field up to the threshold is returned\&. An error is returned for fields which cannot be retrieved\&. See the error list below for details\&.
.PP
\fBsd_journal_enumerate_available_unique()\fR
is similar to
\fBsd_journal_enumerate_unique()\fR, but silently skips any fields which may be valid, but are too large or not supported by current implementation\&.
.PP
\fBsd_journal_restart_unique()\fR
resets the data enumeration index to the beginning of the list\&. The next invocation of
\fBsd_journal_enumerate_unique()\fR
will return the first field data matching the field name again\&.
.PP
Note that the
\fBSD_JOURNAL_FOREACH_UNIQUE()\fR
macro may be used as a handy wrapper around
\fBsd_journal_restart_unique()\fR
and
\fBsd_journal_enumerate_available_unique()\fR\&.
.PP
Note that these functions currently are not influenced by matches set with
\fBsd_journal_add_match()\fR
but this might change in a later version of this software\&.
.PP
To enumerate all field names currently in use (and thus all suitable field parameters for
\fBsd_journal_query_unique()\fR), use the
\fBsd_journal_enumerate_fields\fR(3)
call\&.
.SH "RETURN VALUE"
.PP
\fBsd_journal_query_unique()\fR
returns 0 on success or a negative errno\-style error code\&.
\fBsd_journal_enumerate_unique()\fR
and
\fBsd_journal_query_available_unique()\fR
return a positive integer if the next field data has been read, 0 when no more fields remain, or a negative errno\-style error code\&.
\fBsd_journal_restart_unique()\fR
doesn\*(Aqt return anything\&.
.SS "Errors"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-EINVAL\fR
.RS 4
One of the required parameters is
\fBNULL\fR
or invalid\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-ECHILD\fR
.RS 4
The journal object was created in a different process, library or module instance\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-EADDRNOTAVAIL\fR
.RS 4
The read pointer is not positioned at a valid entry;
\fBsd_journal_next\fR(3)
or a related call has not been called at least once\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-ENOENT\fR
.RS 4
The current entry does not include the specified field\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-ENOBUFS\fR
.RS 4
A compressed entry is too large\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-E2BIG\fR
.RS 4
The data field is too large for this computer architecture (e\&.g\&. above 4 GB on a 32\-bit architecture)\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-EPROTONOSUPPORT\fR
.RS 4
The journal is compressed with an unsupported method or the journal uses an unsupported feature\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-EBADMSG\fR
.RS 4
The journal is corrupted (possibly just the entry being iterated over)\&.
.sp
Added in version 246\&.
.RE
.PP
\fB\-EIO\fR
.RS 4
An I/O error was reported by the kernel\&.
.sp
Added in version 246\&.
.RE
.SH "NOTES"
.PP
All functions listed here are thread\-agnostic and only a single specific thread may operate on a given object during its entire lifetime\&. It\*(Aqs safe to allocate multiple independent objects and use each from a specific thread in parallel\&. However, it\*(Aqs not safe to allocate such an object in one thread, and operate or free it from any other, even if locking is used to ensure these threads don\*(Aqt operate on it at the very same time\&.
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.SH "EXAMPLES"
.PP
Use the
\fBSD_JOURNAL_FOREACH_UNIQUE()\fR
macro to iterate through all values a field of the journal can take (and which can be accessed on the given architecture and are not compressed with an unsupported mechanism)\&. The following example lists all unit names referenced in the journal:
.sp
.if n \{\
.RS 4
.\}
.nf
/* SPDX\-License\-Identifier: MIT\-0 */

#include <errno\&.h>
#include <stdio\&.h>
#include <systemd/sd\-journal\&.h>

int main(int argc, char *argv[]) {
  sd_journal *j;
  const void *d;
  size_t l;
  int r;

  r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
  if (r < 0) {
    fprintf(stderr, "Failed to open journal: %s\en", strerror(\-r));
    return 1;
  }
  r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
  if (r < 0) {
    fprintf(stderr, "Failed to query journal: %s\en", strerror(\-r));
    return 1;
  }
  SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
    printf("%\&.*s\en", (int) l, (const char*) d);
  sd_journal_close(j);
  return 0;
}
.fi
.if n \{\
.RE
.\}
.SH "HISTORY"
.PP
\fBsd_journal_query_unique()\fR,
\fBsd_journal_enumerate_unique()\fR,
\fBsd_journal_restart_unique()\fR, and
\fBSD_JOURNAL_FOREACH_UNIQUE()\fR
were added in version 195\&.
.PP
\fBsd_journal_enumerate_available_unique()\fR
was added in version 246\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1), \fBsystemd.journal-fields\fR(7), \fBsd-journal\fR(3), \fBsd_journal_open\fR(3), \fBsd_journal_enumerate_fields\fR(3), \fBsd_journal_get_data\fR(3), \fBsd_journal_add_match\fR(3)
